Processes for conditioning (also referred to as finishing or recrystallizing) isoindoline pigments are disclosed in, for example, German Laid-Open Application DOS No. 2,628,409, Examples 4 and 5, and DOS No. 2,800,815, Examples 1, 2 and 3, and in EP-A No. 38 548, Examples 1 to 5. The processes are carried out in such a way that the condensate of diiminoisonidoline and barbituric acid (molar ratio 1:2) is heated for several hours at &gt;100.degree. C. in water or a mixture of water and ethylene glycol, depending on the particle size desired.
In order to obtain particularly well crystallized, coarsely crystalline and hence particularly high-hiding and weatherfast pigmentary forms, it is necessary to carry out conditioning under superatmospheric pressure and at temperatures substantially above 100.degree. C., or to accept very long treatment times at below 100.degree. C. Because of the partial decomposition of the isoindoline pigments under these finishing conditions, extremely coarse-particled pigmentary forms, which exhibit very high weatherfastness, can be obtained only with a loss of yield.